Fix gcc7 compile

mysql.cc: In function 'void build_completion_hash(bool, bool)':
mysql.cc:2687:37: error: invalid conversion from 'char' to 'char*' [-fpermissive]
       field_names[i][num_fields*2]= '\0';
                                     ^~~~

Patch was partly backported from upstream commit:
https://github.com/mysql/mysql-server/commit/ae21683d980d5fe9e39bd0193827ea3604256eb9

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

diff -uNr mysql-5.1.73.org/client/mysql.cc mysql-5.1.73/client/mysql.cc
--- mysql-5.1.73.org/client/mysql.cc	2013-11-04 19:52:27.000000000 +0100
+++ mysql-5.1.73/client/mysql.cc	2017-05-13 19:52:10.125101101 +0200
@@ -2684,7 +2684,7 @@
         mysql_free_result(fields);
         break;
       }
-      field_names[i][num_fields*2]= '\0';
+      field_names[i][num_fields*2]= NULL;
       j=0;
       while ((sql_field=mysql_fetch_field(fields)))
       {